My present-day research assesses the causes of tropical cyclones (TC) climate variability, with an emphasis on the role of dust aerosols. I approach the problem through simple model development and a combination of reanalysis data and EOF-like methods. Recently, I have also worked on the thermodynamics of TCs, on theoretical bounds on TC intensity, and on deep convective initiation.
Climate and Tropical Cyclones
I currently work on deepening our understanding of hurricane activity variability by partitioning variations of potential intensity (PI) between global contributions (like CO2 increase) and local contributions (like dust aerosol variability).
The first step in finding the origin of PI variability has been achieved through the development of a linear model for PI variations, in which global and local contributions can be decoupled. This model successfully explains up to 96% of the PI variance in reanalysis data as seen in Fig. 1.
This work showed that over the last 40 years, the effect of local variability on PI has been about 10 times larger than that of global variability. This implies that understanding the variability of regional forcings such as dust radiative forcing is of the utmost importance in order to predict hurricane activity in the Atlantic for the upcoming decades.
Further, the linearity of PI variations shows that all the different local forcings can be studied independently, providing a powerful framework to precisely identify and quantify the causes of observed PI variability. The outcome will help to clarify society’s expectations for near-future hurricane activity variability.
Fundamental Physics of Tropical Cyclones
My previous PhD work focused on expanding our understanding of factors limiting the maximum intensity of TCs, and of TC feedbacks on their immediate environment.
My early PhD research introduced a new form of potential intensity bound on TC surface wind speeds, based on the idea of a differential Carnot cycle. Further, I verified the validity of pre-existing forms of PI, and concluded that reported discrepancies resulted from misapplications of the theory, In other words, all PI bounds are valid so long as they are compared to an appropriate “parent” quantity.
I then studied the influence of TCs on their thermodynamic environment, focusing on explaining the regularly reported long-term intensity decay in axisymmetric numerical models. I found that the decay is due to a drying of the TC’s environment by subsidence, which leads to a decrease in updraft entropy and of work produced by the system. Such a decay is particularly marked in axisymmetric models because they do not represent midlevel moistening mechanisms accurately, which leads to further drying. Indeed, completely dry simulations show no sign of decay, as do artificially moistened storms, highlighting the critical role of subsidence region moisture in models, and perhaps in nature too.
Mid-Latitude Mesoscale Meteorology
The goal of this project was to investigate the parameters that favor the transition of nascent cumuli into deep convection. It was a fun project which required developing a thermal tracking algorithm to follow the life-cycle of simulated convective cores.
The tracking algorithm (in action in Vid. 1) provided cloud-related information like horizontal area, vertical velocity, and core temperature and moisture. This information showed that nascent cloud properties, or “Nature” has predictive power over deep convection initiation, and this relation is influenced by environmental parameters (cloud-layer lapse rate, midlevel humidity, etc.). The main finding of this project was that the width of the incipient cores, by mitigating entrainment effects, allows cores to retain high buoyancy and allows deep convection to initiate. The width of these cores is in turn determined by the magnitude of boundary layer convergence.
(2020) Rousseau-Rizzi, R., R. Rotunno, and G. Bryan: A Thermodynamic Perspective on Steady-State Tropical Cyclones. Journal of the Atmospheric Sciences.
(2020) Emanuel, K., and R. Rousseau-Rizzi: Reply to “Comments on ‘An Evaluation of Hurricane Superintensity in Axisymmetric Numerical Models’”. Journal of the Atmospheric Sciences.
(2020) Rousseau-Rizzi, R., and K. Emanuel: Reply to “Comments on ‘An Evaluation of Hurricane Superintensity in Axisymmetric Numerical Models’”. Journal of the Atmospheric Sciences.
(2019) Rousseau-Rizzi, R., and K. Emanuel: An Evaluation of Hurricane Superintensity in Axisymmetric Numerical Models. Journal of the Atmospheric Sciences.
(2017) Rousseau-Rizzi, R., D. Kirshbaum, and M. K. Yau: Initiation of Deep Convection over an Idealized Mesoscale Convergence Line. Journal of the Atmospheric Sciences.
(2020) A Linear Model for Potential Intensity Variability. AGU Fall Meeting
(2020) Steady-State Tropical Cyclones in Axisymmetric Numerical Models (poster). AMS Annual Meeting
(2019) An Evaluation of Hurricane Superintensity in Axisymmetric Numerical Models. Conference on Atmospheric and Oceanic Fluid Dynamics
(2018) An evaluation of potential intensity in simulated axisymmetric hurricanes. AGU Fall Meeting
(2016) The Transition from Shallow-to-Deep Cumulus Convection Over an Idealized Mesoscale Convergence Zone. 50th CMOS Congress*
* Presented by Prof. Daniel Kirshbaum
(2015) The Shallow-to-Deep Convective Transition Over an Idealized Mesoscale Convergence Zone. AGU Fall Meeting
(2015) The Shallow-to-Deep Convective Transition Over an Idealized Mesoscale Convergence Zone (poster). AMS 16th Conference on Mesoscale Processes